1. Field of the Invention
The present invention relates to an ultrasonic cleaning apparatus for cleaning materials such as a semiconductor substrate and glass substrate at a surface by using ultrasonic waves.
2. Description of the Related Art
A process of manufacturing substrates such as semiconductor substrate and glass substrate includes a cleaning step of eliminating particles adhering to the surface of a substrate. In this cleaning step, an ultrasonic cleaning apparatus is used, which eliminates particles adhering to the surface of the substrate by applying an ultrasonic wave to a cleaning liquid.
Generally, a nozzle type and a slit type are known as an ultrasonic cleaning apparatus. A nozzle type ultrasonic cleaning apparatus cleans a material at a surface while rotating the material on a rotary table, and has a cylindrical cleaning head provided substantially vertical in the upper surface side of the material.
A slit type ultrasonic cleaning apparatus cleans a material at a surface while conveying the material by a conveying unit such as rollers, and has a bar-shaped cleaning head provided horizontally in the upper surface side of the material, so as to cross the material conveying direction.
These cleaning heads has a liquid chamber inside to store a cleaning liquid, a nozzle or slit on the lower surface to eject the cleaning liquid, and an ultrasonic transducer outside the upper surface to apply an ultrasonic wave to the cleaning liquid in the cleaning chamber.
In the ultrasonic cleaning apparatus configured as above has an ultrasonic transducer on the upper surface of the cleaning head, and it is necessary to keep the liquid chamber filled with a cleaning liquid to apply an ultrasonic wave generated by the ultrasonic transducer to the cleaning liquid. Further, it is necessary to keep the liquid chamber filled with a cleaning liquid to prevent deterioration of the ultrasonic transducer by heat.
However, if the liquid chamber is filled with a cleaning liquid, the cleaning liquid more than the necessary quantity is ejected from the nozzle or slit and much cleaning liquid is wasted.
To solve the above problem, an ultrasonic cleaning apparatus has been developed in recent years, (Jpn. Pat. Appln. KOKAI Publication No. 2003-31540).
The ultrasonic cleaning apparatus supplies a cleaning liquid to an upper surface of a material to be cleaned, and applies an ultrasonic wave to the cleaning liquid on the upper surface of the material.
The ultrasonic cleaning apparatus has a cylindrical ultrasonic wave application head. The ultrasonic wave application head is provided substantially vertical to the upper surface of the material, and has an ultrasonic lens as a diffusion application means at the lower end, an ultrasonic transducer to generate an ultrasonic wave inside, and a nozzle to supply a cleaning liquid to the upper surface of a material outside.
The ultrasonic lens transmits the ultrasonic wave generated by the ultrasonic transducer to the cleaning liquid on the upper surface of the material, and has inside a path to flow a cooling liquid to cool the ultrasonic transducer. The lower surface of the ultrasonic lens is curved to swell toward the upper surface of the material to diffuse and apply the ultrasonic wave generated by the ultrasonic transducer to a wide area of the cleaning liquid.
According to the ultrasonic cleaning apparatus configured as above, ultrasonic wave is applied directly from the ultrasonic lens to the cleaning liquid supplied to the surface of the material. Therefore, the material can be cleaned with the least minimum cleaning liquid, and the amount of the cleaning liquid with which the material is cleaned can be decreased. Further, the ultrasonic transducer can be cooled by flowing a cooling liquid in the path provided in the ultrasonic lens.
As an ultrasonic cleaning apparatus of the type similar to the above, there is a known ultrasonic cleaning apparatus, in which a cylindrical ultrasonic lens is provided parallel or substantially parallel to a material to be cleaned.
One end of the ultrasonic lens is expanded in the diameter, and the expanded end face has a block made of material with high thermal conductivity such as copper. A path to flow a cooling liquid is formed inside the block, and an ultrasonic transducer is provided on the outside surface of the block.
In this ultrasonic cleaning apparatus, an ultrasonic wave generated by the ultrasonic transducer is transmitted to the ultrasonic lens through the block, and vibrates the ultrasonic lens laterally or in the direction crossing the axial line. This lateral vibration is transmitted to the cleaning liquid between the ultrasonic lens and the material, and used to clean the material at an upper surface.
However, if an ultrasonic wave is applied to a cleaning liquid by using an ultrasonic lens as described above, an ultrasonic wave generated by an ultrasonic transducer may reflect at an upper surface of a material to be cleaned and enter an ultrasonic transducer through an ultrasonic lens.
When an ultrasonic wave enters an ultrasonic transducer, the ultrasonic transducer is extremely heated exceeding the cooling effect of a cooling liquid then, the polarization of the ultrasonic transducer may be deteriorated. In this case, the ultrasonic transducer cannot generate an ultrasonic wave with a constant intensity, and decreases the cleaning efficiency.
On the other hand, in the ultrasonic cleaning apparatus in which a bar-shaped ultrasonic lens is provided parallel to a material to be cleaned and an ultrasonic transducer is provided at one end of the lens, an ultrasonic wave transmission route is not straight, and an ultrasonic wave reflected on the material hardly reaches the ultrasonic transducer.
However, in this ultrasonic cleaning apparatus, the ultrasonic wave generated by the ultrasonic transducer is applied to the cleaning liquid after converting to vibration in the lateral direction of the ultrasonic lens, and the intensity of the ultrasonic wave applied to the cleaning liquid is decreased. Further, since the vibration in the lateral direction of the ultrasonic lens advances radially around the axial line of the ultrasonic lens, or toward all directions of the radius of the ultrasonic lens, the ultrasonic vibration toward the upper side where no cleaning liquid exists is not effectively used for cleaning the material.